Evidence for Magnetic Reconnection as the Precursor to Discrete Aurora at Mars

Discrete aurora at Mars are characterized as localized, short-lasting ultraviolet emissions on the nightside. They are caused by the precipitation of accelerated electron along open magnetic field lines and their collision with the Martian atmosphere. Discrete auroral emissions detected by the Imaging Ultraviolet Spectrograph (IUVS) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft occur most frequently over the strongest crustal magnetic anomalies at Mars and under particular local time and upstream interplanetary magnetic field (IMF) conditions. These trends suggest that the onset of discrete aurora is controlled by the processes that govern the interaction between the draped IMF and the crustal magnetic anomalies. Here, we analyze MAVEN magnetometer measurements over regions of strong crustal fields during 49 discrete auroral events observed by IUVS. Our results indicate that the draped IMF orientations during each discrete auroral event show a clear tendency to be anti-parallel to the underlying crustal anomaly magnetic fields near the location of the emission onset. This suggests that magnetic reconnection, a process that favors anti-parallel magnetic field regimes, between the crustal fields and the draped IMF plays a role in discrete aurora formation. Of the 49 discrete auroral events analyzed, 42 (86%) occurred under draped IMF conditions that are susceptible to reconnection with the underlying crustal anomalies. This investigation produces strong evidence linking discrete aurora to magnetic reconnection at Mars and provides insights into other trends in discrete auroral activation such as local time and upstream IMF conditions. There are regions on the surface of Mars that possess strong magnetic fields. Ultraviolet aurora known as "discrete aurora" at Mars are observed most frequently over a region of these strong crustal fields and are caused by the energetic collision between charged particles and the Martian atmosphere. These particles are thought to be the result of the interaction between Martian magnetic fields and the flow of charged particles and interplanetary magnetic field (IMF) known as the solar wind. Our work analyzes magnetic field observations from the magnetometer onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft along orbits in which discrete aurora were detected over a region of strong crustal magnetism. We find that discrete auroral emissions over these strong crustal magnetic fields occur most frequently when the surrounding draped IMF conditions are susceptible to undergoing a process called "magnetic reconnection," in which magnetic field lines are reassigned into a different magnetic connectivity. This is the first study to definitively link the process of discrete aurora to magnetic reconnection and provides a framework for understanding previously unexplained trends in discrete aurora activation at Mars. Discrete aurora over two regions of crustal fields at Mars occur under specific draped magnetic field conditionsOf the 49 discrete aurora analyzed, 42 (86%) occur under conditions that favor anti-parallel magnetic reconnectionReconnection plays a role in discrete aurora onset, explaining trends in auroral detections including local time and upstream conditions